The present invention relates to the art of diagnostic imaging. In particular, it relates to positron emission tomography (PET) and other diagnostic modes in which a subject is examined and an image of the subject is reconstructed from information obtained during the examination.
Previously, PET has been used to study a radionuclide distribution in subjects. Typically, one or more radiopharmaceuticals (i.e., tracers) are injected into a subject. The radiopharmaceuticals are commonly injected into the subject's blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. PET is a physiologic imaging modality that images the distribution of radiolabeled tracers within the body. Unlike anatomic imaging modalities, which image tissue structures and morphology, PET can characterize the functional, metabolic, and physiologic status of tissues in vivo. Hundreds, if not thousands, of radiotracers have been investigated for PET, targeting parameters such as glucose metabolism, blood flow, hypoxia, cellular proliferation, amino acid synthesis, gene expression, and so on. As more is learned about the molecular bases for disease and treatment, PET becomes an increasingly powerful modality for characterizing and monitoring disease.
In some instances a subject must undergo multiple injections of a tracers and scans associated with each tracer. Subsequent injections may be the same tracer as the first, or they may each be a different tracer. Prior to each injection sufficient time must elapse to allow the earlier introduced tracer to flush from the subject or to decay. This decreases throughput of patients and is inconvenient for patients in clinical applications. To alleviate some of these challenges, rapid multi-tracer PET has been investigated. For instance, rate parameters for individual tracers have been recovered from data with overlapping signals from different PET tracers based on different half-lives, tracer kinetics, or both (Huang et al. 1982; Koeppe et al 1998 and 2001; Converse et al. 2004 and Kadrmas and Rust 2005). In 1982, Huang et al. demonstrated in a phantom that, when imaging static distributions of multiple PET tracer with different half-lives, images of each tracer can be recovered based on their different rates of radioactive decay. In short, this amounts to treating the dynamic PET signal as a sum of exponentials with known decay constants and estimating the coefficients of each exponential. While an important contribution, this approach has little or no practical application because (i) PET tracers are rarely static, except for irreversible tracers long after injection; and (ii) separation of summed exponentials is a poorly conditioned problem sensitive to statistical noise—requiring long scan durations relative to the half-lives of the tracers used in order to get acceptable results. In 1998, Koeppe et al. recovered kinetic rate parameters for two 11C-labeled brain tracers injected 10-30 minutes apart with a single dynamic PET scan. Though the multi-tracer PET signal was not separated into individual tracer components in this work and images of each tracer were not recovered, it did demonstrate recovery of certain rate parameters from a dual-tracer dataset.
However, these earlier works have not resulted in the recreation of activity timecourses for each individual tracer from combine tracer data nor have they allowed the creation of an image from combined tracer data obtained in a single scan.
Therefore, what is needed is a means to overcome challenges found in the art, some of which are described above.